DSP simulation

dspsim.py

@@ -0,0 +1,79 @@
+import matplotlib
+matplotlib.use('TkAgg')
+
+import numpy as np
+from scipy import signal
+import matplotlib.pyplot as plt
+
+
+class LinearPhaseFilter:
+    def __init__(self, numtaps, *args, **kwargs):
+        self.numtaps = numtaps
+        self.coef = signal.firwin(numtaps, *args, **kwargs)
+        self.state = np.zeros(self.numtaps - 1)
+
+    def input(self, block):
+        output, self.state = signal.lfilter(self.coef, [1.0], block, zi=self.state)
+        return output
+
+    def delay(self):
+        assert(self.numtaps % 2 == 1)
+        return (self.numtaps - 1)//2
+
+
+fs = 192000
+t = np.arange(0.0, 0.15, step=1./fs)
+path = 0.1e-3*np.sin(2.*np.pi*47.*t)
+interferogram = np.sin(2*np.pi*path/1550e-9) + np.sin(2*np.pi*path/1270.0e-9)
+
+filt = LinearPhaseFilter(8191, [80., 120.], pass_zero=False, fs=fs)
+demod = filt.input(np.diff(interferogram)**2)
+
+d = filt.delay() + 3000
+t = t[d:]
+path = path[d:]
+interferogram = interferogram[d:]
+demod = demod[d:]
+
+zero_crossings = np.where(np.diff(np.sign(demod)))[0]
+x = []
+y = []
+for i, zero in enumerate(zero_crossings):
+    x.append(t[zero])
+    y.append(i*np.pi)
+A = np.vstack([x, np.ones(len(x))]).T
+m, c = np.linalg.lstsq(A, y, rcond=None)[0]
+normspeed = (1. + np.sin(m*t+c))/2.
+
+segments = []
+i = 0
+try:
+    while True:
+        while normspeed[i] < 0.25:
+            i += 1
+        segment = []
+        nco = 0.0
+        while normspeed[i] >= 0.25:
+            nco += normspeed[i]
+            if nco >= 1.0:
+                nco -= 1.0
+                segment.append(interferogram[i])
+            i += 1
+        segments.append(segment)
+except IndexError:
+    pass
+
+
+
+fig, axs = plt.subplots(3+len(segments))
+axs[0].plot(path)
+axs[1].plot(interferogram)
+axs[2].plot(demod)
+
+for i, segment in enumerate(segments):
+    print(i, len(segment))
+    axs[3+i].plot(np.abs(np.fft.rfft(segment*signal.blackmanharris(len(segment)))))
+
+
+plt.show()
+